[eng] Nowadays, climate change is the most important threat to our global
environment, with global warming as one of its main consequences. The
Mediterranean area is especially sensitive to this warming and its marine biodiversity is
undergoing rapid alteration and “tropicalization”. In this study we have experimentally
evaluated response of the metabolic rates as Community Respiration (CR), Gross
Primary Production (GPP) and Net Community Production (NCP), of seagrass
communities consisting of Cymodocea nodosa colonized by the invasive macroalgae
Halimeda incrassata. We assessed two different seasons with their ambient
temperature ranges and temperature increases as expected towards the end of the
21st century in the Mediterranean. The metabolic rates were obtained from changes in
oxygen concentration measured with oxygen-sensors (IKS-Aquastar) in incubation
chambers at temperatures ranging from 17ºC to 23ºC in spring and from 25ºC to 33ºC
in summer. Using point measurements of alkalinity and continuous pHNBS values we
also obtained the pCO2 (μatm) concentration during this incubations.
Metabolic rates are sensitive to temperature increments, especially in summer,
while during the spring the invasive macroalgae seem inactive. At higher temperatures
there is an increase of CR and GPP when macrophytes were evaluated separately.
This increase is higher for CR than for GPP. In invaded meadows, with both species
present, the presence of Halimeda incrassata causes the reduction of metabolic rates
starting from an optimum temperature. With temperature increases, these communities
tend to shift from a positive NCP of an autotrophic community, to a negative NCP of a
heterotrophic community, with a tendency to emit CO2 and exhaust the oxygen
concentration of the water column. In summary, the combined effect of a temperature
increase and colonization by invasive species reverts these ecosystems to a carbon
source instead of sink, with its consequent negative effects for the environment.